1. Field of the Invention
The present invention relates to a rear wheel steering device in which a rear wheel is steered in accordance with a front wheel.
2. Description of the Related Art
As commonly known, a so called 4 wheel steering (4WS) vehicle is provided with a rear wheel steering device for steering the rear wheel in accordance with a steering wheel operation as well as a front wheel steering device.
In the rear wheel steering device, it has been generally proposed that the rear wheel be steered in the opposite direction to the front wheel when the vehicle speed is lower than a predetermined value (speed responsive type) or when the steering angle for the front wheel is greater than a predetermined value (steering angle responsive type).
In the 4WS vehicle, the rear wheel steering characteristic is determined so as not to produce so called head-in and head-out of the vehicle in which the vehicle head portion significantly comes off from the turning path of the vehicle. In other words, the rear wheel steering characteristic is determined such that a longitudinal center line of the vehicle moves along a tangential line of the cornering path of the vehicle wherein a vehicle body slip angle .beta. is zero.
A conventional rear wheel steering device can provide a cornering characteristic in which vehicle body slip angle .beta. is zero in a steady cornering condition after a steering operation. When the driver operates the steering wheel for cornering, the vehicle experiences a transitional cornering condition just after the steering operation by the driver before the vehicle reaches the steady cornering condition. In the transitional cornering condition, the vehicle is not subjected to a side force high enough to cause the vehicle body to be moved substantially perpendicular to the tangential line of the cornering path.
Thus, the vehicle moves obliquely from the tangential line of the cornering path at the initial stage of the cornering movement. This affects the driving feeling of the driver badly.
In view of this, it has been proposed that the rear wheel be steered in a direction opposite to the front wheels (reverse phase control) for a moment just after the steering operation by the driver. This facilitates a responsive yawing of the vehicle and produces a proper cornering force even in the initial stage of the cornering movement. Therefore, it is possible for the vehicle to move along the cornering path in the transitional condition as well as the steady cornering condition.
Meanwhile, it has been proposed that the 4WS be provided with a fail safe system in which an abnormality is detected when a steering ratio of the rear wheel to the front wheel is increased beyond a predetermined zone; such a system is proposed in Japanese Patent Publication No. 2-43674. The fail safe system functions to detect the abnormality of the rear steering device quickly to improve the safety of the 4WS vehicle, for example by switching from the 4WS condition to 2WS condition.
In the conventional fail safe system for the rear wheel steering device, generally, the rear wheel steering is prohibited in the direction opposite to the front wheel when the vehicle speed is relatively high or when the steering angle of the front wheel is relatively small. On the other hand, under the reverse phase control, the rear wheel steering in the opposite direction to the front wheel steering is prohibited irrespective of the vehicle speed and the front wheel steering angle as far as the vehicle is in the transitional cornering condition which occurs for a certain period just after the steering operation is made. Therefore, where the rear wheel steering device is provided with both the conventional fail safe system as mentioned above and the reverse phase control system, there is a fear that the fail safe system will detect erroneously an abnormality of the rear wheel steering device when the rear wheel is steered in the direction opposite to the front wheel in accordance with the reverse phase control.